More generally, a roll stand comprises a set of overlapping rollers within a rigid frame. For instance, in a "Quarto" assembly, the rolling mill comprises two working cylinders, which are associated, respectively, with two backing cylinders and whose opposite generatrices define a roll gap for the product to be rolled. Clamping means, supported by the rigid frame, determine crushing of the product whose thickness, at the rolling mill entrance, is greater than the roll gap.
The product to be rolled, whose shape is a bar or a plane slab, is conveyed normally to the stand, via a input roller table comprising several rollers with parallel axes which define a supporting plane of the slab, more or less at the level of the roll gap. At least a few of these rollers are rotary-driven in order to cause, by a friction effect, the forward motion of the slab towards the working cylinders. A second roller table, located at the exit side, receives the product after rolling.
In so-called "reversing" rolling mills, the roll stand is associated with two roller tables located on both sides and used alternately as infeed table and reception table, depending on the flow direction of the product.
Since the product is thicker before going through the mill, the supporting plane defined by the rollers of the table located upstream must normally be above the upper generatrix of the lower working cylinder and at a distance substantially equal to half the thickness reduction, in such a manner that the medial horizontal plane of the slab coincides substantially with the medial plane of the roll gap situated at equal distance from both cylinders.
Conversely, the supporting plane defined by the rollers of the exit table should, theoretically, be located at the level of the upper generatrix of the lower working cylinder, in order to accommodate the rolled product when it leaves the roll gap. To avoid any risk of jamming, the exit table is preferably slightly lowered with respect to the lower cylinder, but the consequence is that the nose of the rolled slab is liable to fall onto the exit rollers and may be damaged.
It has thus been suggested, in a particular embodiment described in JP-A-30.35805, to reinforce the nose of the plate slab by giving it an undulated shape. To this end, the last roller of the input table is raised above the normal level, using hydraulic jacks supported by two chocks supporting the rollers' extremities. In this way, when entering the slab into the roll gap, the nose can be bent upwards, then downwards once the slab has been entered properly. At this moment, the last roller is brought back to its normal level to complete the pass.
However, the consequence of this technique is that the movable roller as well as its chocks must support large loads in order to cause the two bends of the slab. Moreover, the nose of the slab must be bent again to pass into the roll gap, and this makes entry more difficult.
Still, more generally speaking, the entry of the slab into the roll gap may prove difficult, and this causes its thickness to be reduced to a lesser extent than would be allowed, theoretically, by the power of the rolling mill.
Indeed, when the product is already engaged between the working cylinders, the crushing action exerted by these cylinders generates frictions which are sufficient to move the product forward.
Conversely, during the entry phase into the roll stand, in order to reduce the thickness significantly, the frictions exerted by the cylinders on the edges of the slab may be insufficient to drive the product and it may be necessary to push it and help it to engage into the cylinders.
In order to enhance the entry of the slab, it can be given a certain velocity before contacting the cylinders, in order to benefit from an inertia effect, but such an effect remains limited. Quite frequently, the slab will not engage, and this may call for reducing its thickness to less than the desired value, allowed by the power of the rolling mill. The number of passes is thus increased.
To obviate this drawback, it is not sufficient to increase the torque applied to the driven rollers, since the thrust load exerted on the slab depends on the friction between the slab and the rollers.
In studying the conditions of passage of the slab through the roll gap, it has been noted that, after a few passes, the slab under rolling becomes slightly bent and, consequently rests, in practice, on the table only by its edge facing the roll stand, whereas the thrust action is often exerted by only one roller.